1. Field of the Invention
The present invention relates to a multi-layer optical recording medium having a plurality of recording layers, and more particularly to a multi-layer optical recording medium having data recording regions divided by preformatted regions and a method of manufacturing the recording medium.
2. Description of the Related Art
A multi-layer optical disc is recently known as a large capacity recording medium that has an increased recording capacity per one side of the optical disc. Such multi-layer optical disc has a structure in which a plurality of recording layers are stacked at relatively small gaps. Another type of conventional multi-layer optical disc is a rewritable multi-layer optical disc employing a recording material or medium such as a phase change medium.
For simplicity of description, the following description deals with a two-layer DVD (Digital Versatile Disc) having two recording layers in which each layer contains the phase change medium. More particularly, the two-layer DVD has a structure in which phase-change recording films are formed on both layers, that is, an upper layer (or first recording layer) which is proximate to an object lens of an optical pickup and a lower layer (or second recording layer). A laser beam is focused on one of the recording layers and a signal is recorded on or reproduced from the recording layer when information is recorded on or reproduced from the two-layer disc.
It should be assumed now that the laser beam is focused on the second recording layer (hereinafter, simply referred to as “second layer”) of the above described two-layer DVD or the like upon recording or reproduction. In this case, the laser beam is transmitted through the first recording layer (hereinafter, simply referred to as “first layer”) and is radiated on the second layer to record and/or reproduce a data signal. Such recording and/or reproduction (hereinafter, simply referred to as “recording/reproduction”) is influenced by the first layer if an amount of light reflected from the first layer and received by a light receiving unit varies and/or if an amount of light transmitted to the second layer through the first layer varies. An adverse effect caused by the variations in the amount of reflected light from the first layer can be reduced by modifying a structure of a light detecting system or other element(s). The variations in the amount of transmitted light through the first layer, however, remain in the form of variations in intensity of the recording light during the recording operation, and in the form of variations in level of a reproduced signal during the reproducing operation. These adverse effects are sometimes not negligible.
When a phase change medium such as germanium antimony tellurium (GeSbTe) is used for the recording layer, optical transmittance of a crystal portion of the medium differs from that of an amorphous portion; the former is lower than the latter. Almost 100% of the areas of the phase change medium which have no data recorded thereon are crystal portions, while crystal portions and amorphous portions exist in the recorded areas in a mixed manner. Further, the transmittance for the beam is an average value of the transmittance of the crystal portion and that of the amorphous portion although the beam is not focused on the recorded area of the first layer and the recorded signal is not reproduced therefrom. As a result, upon reproduction of the second layer, the amount of received light (i.e., the RF signal level during the reproducing operation) differs according to whether the beam passes through the recorded area or the non-recorded area of the first layer.
If the ratio of the crystal area to the amorphous area is constant in the region of the first layer through which the beam passes, the transmittance of the beam does not change so that the amount of received light (i.e., RF signal level) does not change.
A general rewritable multi-layer optical disc, however, is provided with a preformatted area in which no data signal is recorded. Referring to FIGS. 1 and 2 of the accompanying drawings, a DVD-RAM (Random Access Memory) will be taken as an example. The disc 3 has recording areas divided by preformatted areas 5 in a tracing direction (i.e., circumferential or tangential direction of the disc) and a plurality of data areas 6 are concentrically formed. A sector 7 is defined by a preformatted area 5 and an adjacent data area 6.
FIG. 2 illustrates an enlarged view of the preformatted area and the neighboring areas (portion “A” in FIG. 1) together with transmittance of these areas. Information data such as an addresses is recorded in the form of embossed pits 8 within the preformatted area 5. The data area 6 includes lands (L) and grooves (G), and recorded marks 9 are formed on data recorded portions.
As illustrated in FIG. 2, the average transmittance TD of the data area 6 is greater than the transmittance TP of the preformatted area 5. Thus, the transmittance changes in the preformatted area 5 of the first layer and the recording beam intensity or reproduced signal intensity changes when the recording operation or reproducing operation is performed for the second layer. In order to avoid the adverse effect from occurring in the multi-layer optical recording medium having the preformatted areas during information recording and/or reproduction, the positions of the preformatted areas in the first layer should be aligned with those of the preformatted areas in the second layer as shown in FIG. 3. No adverse influence occurs during the recording and/or reproduction operations if the preformatted areas of the second layer always lie below the preformatted areas of the first layer that would reduce the amount of transmitted light. It is, however, practically difficult to align the positions of the preformatted areas with each other in a manufacturing process.
There is therefore a demand for a method of manufacturing an optical disc having the preformatted areas of the first layer aligned with those of the second layer, or an optical disc that has minimized location deviations of the preformatted areas between the first and second layers. It is also necessary that the adverse effects in recording/reproduction operations such as an S/N ratio (signal to noise ratio) deterioration and reproduced RF signal variations resulting from the above described transmittance variations are prevented even if the preformatted areas of the manufactured disc are not aligned with each other between the recording layers.